Effects of plant diversity on primary production and species interactions in brackish water angiosperm communities

Research on plant biodiversity and ecosystem functioning has mainly focused on terrestrial
ecosystems, and our understanding of how plant species diversity and interactions affect processes
in marine ecosystems is still limited. To investigate if plant species richness and composition
influence plant productivity in brackish water angiosperm communities, a 14 wk field experiment
was conducted. Using a replacement design with a standardized initial aboveground biomass, shoots
of Zostera marina, Potamogeton filiformis and P. perfoliatus were planted on a shallow, sandy bottom
in replicated monocultures and all possible species combinations. Response variables included
aboveground and belowground biomass, shoot density, space occupation and porewater nutrients.
To determine whether selection and/or complementarity controlled productivity, additive partitioning
and Di were calculated. Richness effects were species-specific and only increased the biomass
production of P. perfoliatus and tuber production of P. filiformis, while species composition generally
had a stronger effect on biomass production. Additive partitioning indicated a positive complementarity
effect for the aboveground biomass production in bicultures in general, while a positive net
effect was found in the P. perfoliatus and P. filiformis biculture. Despite the absence of significant
results for other treatments and plant variables, a trend of positive complementarity and negative
selection effects were present. Plant diversity had no clear effect on sediment porewater nutrient
concentrations, indicating weak richness effects on resource use.